Process for preparing acicular iron oxide particles

Abstract

This disclosure relates to a process for preparing acicular magnetic iron oxide particles by (a) adding an aqueous solution of a ferrous salt to an aqueous solution of a stoichiometric excess of alkaline hydroxide while substantially avoiding local excesses of the ferrous salt, to form an aqueous dispersion of ferrous hydroxide particles, (b) oxidizing these particles to form non-magnetic alpha ferric oxide hydrate particles, and (c) converting the non-magnetic particles to magnetic iron oxide particles by dehydration and reduction or dehydration, reduction and oxidation. The ferrous salt solution is subjected to reducing conditions prior to and/or during the addition to the alkaline hydroxide solution to reduce contaminating ferric ions that are normally present to ferrous ions.

What is claimed is: 
     
       1. In a process for preparing acicular magnetic iron oxide particles comprising (a) adding an aqueous solution of ferrous salt to an aqueous solution of a stoichiometric excess of alkaline hydroxide while substantially avoiding local excesses of said ferrous salt, to form an aqueous dispersion of ferrous hydroxide particles, (b) oxidizing said particles to form non-magnetic alpha ferric oxide hydrate particles, and (c) converting said non-magnetic particles to magnetic iron oxide particles by dehydration and reduction or dehydration, reduction and oxidation, the improvement wherein said ferrous salt solution is subjected to reducing conditions prior to or during said addition to reduce contaminating ferric ions present therein to ferrous ions. 
     
     
       2. A process according to claim 1 in which said reducing conditions are provided by adding said ferrous salt solution to said alkaline hydroxide solution in the presence of a sufficient concentration of reducing agent to reduce said ferric ions to ferrous ions. 
     
     
       3. A process according to claim 2 in which said reducing agent is an inorganic reducing agent. 
     
     
       4. A process according to claim 3 in which said reducing agent is sodium sulfite or sodium hydrosulfite. 
     
     
       5. A process according to claim 3 in which said reducing agent is added to said aqueous solution of ferrous salt in a concentration in the range of about 10 to 100 meq. per mole of said ferrous salt. 
     
     
       6. A process according to claim 2 in which said reducing agent is a sulfite, hydrosulfite, dihydroxybenzene, or hydroxylamine. 
     
     
       7. A process according to claim 2 in which said reducing agent is added to at least one of said aqueous solutions of ferrous salt or alkaline hydroxide. 
     
     
       8. A process according to claim 2 in which said reducing agent is sodium sulfite, sodium hydrosulfite, hydroquinone or hydroxylamine hydrochloride and is added to said aqueous solution of alkaline hydroxide in a concentration in the range of about 2 to 12 meq. per liter. 
     
     
       9. A process according to claim 2 in which said stoichiometric excess is in the range of about 50 to 100 percent. 
     
     
       10. A process according to claim 9 in which said reducing agent is a sulfite, hydrosulfite, dihydroxybenzene or hydroxylamine. 
     
     
       11. A process according to claim 9 in which said reducing agent is sodium sulfite or sodium hydrosulfite and is added to said aqueous solution of ferrous salt. 
     
     
       12. A process according to claim 1 in which said stoichiometric excess of alkaline hydroxide does not exceed 250 percent. 
     
     
       13. A process according to claim 12 in which said alkaline hydroxide is an alkali metal hydroxide. 
     
     
       14. A process according to claim 13 in which said alkali metal is sodium or potassium and said reducing agent is added to said aqueous solution of ferrous salt. 
     
     
       15. A process according to claim 13 in which said ferrous salt is ferrous sulfate or ferrous chloride. 
     
     
       16. A process according to claim 12 in which said non-magnetic particles are dehydrated, reduced and oxidized to form magnetic gamma ferric oxide particles. 
     
     
       17. A process according to claim 1 including the additional step of doping said magnetic particles with doping metal ions. 
     
     
       18. A process according to claim 17 in which said doping metal ions are present in said aqueous solution of ferrous salt. 
     
     
       19. A process according to claim 17 in which said doping metal ions are cobalt ions. 
     
     
       20. A process according to claim 17 in which said doping metal ions are ions of cobalt, ferrous iron, nickel, chromium, zinc, cadmium, manganese, calcium, tin, samarium, or europium. 
     
     
       21. A process according to claim 20 in which said doping metal ions are ions of cobalt and calcium. 
     
     
       22. A process according to claim 1 in which said non-magnetic particles are dehydrated and reduced to form magnetic ferrous ferric oxide particles. 
     
     
       23. A process according to claim 1 in which said non-magnetic particles are dehydrated, reduced and oxidized to form magnetic gamma ferric oxide particles.